A Long Term Evolution (LTE) system is based on an orthogonal frequency division multiplexing (OFDM) technology, and time-frequency resources of the LTE system are classified into an OFDM symbol in a time domain dimension and an OFDM subcarrier in a frequency domain dimension. A smallest resource granularity is referred to as a resource element (RE), that is, a time and frequency grid representing an OFDM symbol in a time domain and an OFDM subcarrier in a frequency domain.
Service transmission in the LTE system is based on base station scheduling. Generally, a base station sends a control channel. The control channel may carry scheduling information of a data channel of uplink or downlink data. The scheduling information includes control information such as resource allocation information and an encoding adjustment manner. User equipment (UE) receives a downlink data channel or sends an uplink data channel according to the scheduling information carried in the foregoing control channel. The base station schedules the user equipment by using a resource block (RB) as a granularity. One resource block occupies a length of one subframe in a time domain and occupies a width of 12 OFDM subcarriers in a frequency domain. In a case of a common cyclic prefix (CP), one subframe includes 14 OFDM symbols. In a case of an extended CP, one subframe includes 12 OFDM symbols.
To maintain the foregoing service transmission, and perform cell selection, reselection, addition, deletion, handover, or another process, the user equipment needs to perform synchronization, channel state measurement, and radio resource management measurement according to a reference signal sent by the base station.
Synchronization is further classified into initial coarse synchronization and fine time-frequency tracking synchronization. The initial coarse synchronization is implemented according to a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) that are sent by the base station. The fine time-frequency tracking synchronization is implemented by using a cell-specific reference signal (CRS) sent by the base station.
The channel state measurement includes channel measurement and interference measurement, and measurement may be performed based on a CRS or a channel state information reference signal (CSI-RS).
The radio resource management measurement includes measurement of reference signal received power (RSRP), reference signal received quality (RSRQ), a received signal strength indicator (RSSI), or the like, and is currently implemented by using a CRS or a reduced CRS (RCRS). The RSRP indicates average power, which is included in a CRS resource element of a target measured cell, of CRSs sent by the measured cell. The RSSI indicates average power of all signals in OFDM symbols in which the CRSs of the measured cell are located, including signal power of a current cell, signal power of an intra-frequency neighboring cell, signal power leaked from an inter-frequency band to a current frequency band, and average power of all signals such as a thermal noise signal. The RSRQ is obtained according to a ratio of the RSRP to the RSSI, and RSRP measurement and RSSI measurement that determine the RSRQ are performed in OFDM symbols, in which CRSs are located, in a same resource block. The radio resource management (RRM) measurement may further include signal to interference plus noise ratio (SINR) measurement. For example, a signal to interference plus noise ratio may be determined by using a ratio of the RSRP and interference measurement of a neighboring cell. A CRS is sent in each subframe in the LTE system. Subsequently, another state of a new carrier type (NCT) or a current carrier type may be introduced, where a feature is that for a quantity of subframes in which the CRS is sent once, the quantity is increased to five subframes. The CRS in this case may be referred to as a reduced CRS (RCRS).
A subsequent evolved LTE system has a relatively high requirement on power and efficiency of a base station, and in future network topology evolution, an operator deploys a large quantity of heterogeneous networks. In a mainstream deployment scenario, a large quantity of small cells are deployed in a range of a macro cell, where the macro cell mainly provides coverage and a real-time data service, a small cell mainly provides a high-speed data service, and the macro cell and the small cell may be deployed on a same frequency or different frequencies, but a deployment scenario of different frequencies predominates.
In the foregoing heterogeneous network with dense small cells, if all the small cells are turned on, even if there is no service transmission, the foregoing reference signals such as the PSS, the SSS, the CRS/RCRS, and the CSI-RS also need to be sent. Sending of these reference signals having relatively short sending periods (where the sending period of the PSS/SSS/RCRS is five subframes, the sending period of the CRS is one subframe, and the sending period of the CSI-RS is at least five subframes) may cause severe inter-cell interference. In addition, network density may result in that no user equipment is served in a range of a large quantity of small cells, that is, within a particular period of time, user equipment in a range of only some small cells of all the foregoing small cells is served. In this way, a potential solution is to turn off these small cells that serve no user equipment, that is, none of a PSS, an SSS, a CRS/RCRS, a CSI-RS, a control channel, and a data channel is sent, so as to completely turn off the small cells, and achieve an effect of energy saving and reducing inter-cell interference, enabling a small cell that is serving user equipment to provide a more highly efficient service.
However, to completely turn off the small cell also has limitations. For example, the user equipment cannot promptly discover or detect deployment of the small cell and perform radio resource management (RRM) measurement on the small cell. On the other hand, a network side does not know when to turn on the small cell, and cannot determine, according to a result, reported by the user equipment, of measurement performed on the small cell, whether to turn on the small cell and whether to configure the cell for the user equipment.